The gasification of carbonaceous material can produce primarily carbon monoxide and hydrogen, a mixture referred to as syngas or synthesis gas. Carbon dioxide, water and various hydrocarbons can be abundant side products in the gasification product. Depending on the source and composition of the carbonaceous raw material and gasification conditions, the levels of side products and derivatives present as impurities can vary, influencing refining strategies.
During gasification, the sulfur components originated from biomass can be mainly converted to hydrogen sulfide (H2S) and carbonyl sulfide (COS). In comparison to coal gasification, gasifying biomass raw material can produce very low levels of sulfidic, relatively low levels of nitric and low levels of ash impurities. The level of carbon dioxide can be higher than in coal gasification. These impurity levels can still be harmful for further chemical processing and it can desirable to purify the gas. The decrease of hydrogen sulfide concentration can be desirable for the functioning of the catalysts later in the refining of the syngas. On the other hand, the carbon dioxide's role in further reactions can be as an inert. The reason for removing CO2 can relate to optimizing the streams and decreasing volumes of recycle flows and equipment. The strategies employed from coal gasification may not be readily applicable.
Together carbon dioxide, hydrogen sulfide and carbonyl sulfide can be referred to as acid gas since they dissolve in water forming acids. One means for gas purification is absorption, which can been used for acid gas removal from natural and synthesis gases. When purifying biomass originated synthesis gas, absorption with a liquid solvent can be more efficient than solid absorption. For physical absorption, organic solvents at cold temperatures and high pressure can be common. A better washing effect can be attained, for example, by employing a higher pressure, a colder temperature and a higher purity of the absorbent. For chemical absorption, solutions of arsenic salts, various amines and carbonates can be employed. The absorbent can be regenerated by elevating the temperature and/or releasing the pressure.
An effective absorbent for removing acid gas can be, for example, methanol. Methanol employs low temperatures to be efficient and to reduce or avoid absorbent loss. A commercial process using methanol is an acid gas removal process available under the trade name Rectisol®. The Rectisol acid gas removal process does not require hydrolysis of COS to H2S and can reduce sulfur component contents to relatively low levels in syngas. Methanol can have a high affinity for hydrocarbons as well as for acid gas. It can also exhibit capabilities to remove not only sulfur components and CO2 but also many relevant trace components (carbonyles, HCN), which can make Rectisol wash a useful process. The syngas can then be reheated to about 350° C. and passed through a fixed bed of sorbent for sulfur components, such as a ZnO guard bed, to further reduce the sulfur component contents in the syngas. Large temperature differences between process phases can consume a large amount of energy and can make processing expensive.
EP 2223889 discloses a device providing further development of the multistage methanol wash as a part of Integrated Gasification Combined Cycle, IGCC. With the device disclosed, as a multistage process, this version of Rectisol process can remove CO2 from the gas. As a process related to power production, desired purity levels, however, can be different from those applied in chemical or fuel production wherein higher purity can be demanded.
U.S. Patent Application Publication No. 2010/0163803 discloses a process for the production of gas products from a raw synthesis gas that is obtained by gasification of carbon and/or heavy oil. Origin of the gas can give it a characteristic component profile. The process description discloses how both the shifted and the unshifted gas streams can be purified of sulfur components and CO2 in sour gas washing, for example, a cryogenic methanol washing. An apparatus suitable for the process is disclosed as well. Sulfur components and CO2 can be removed together, the washes providing no separation of these components.
In addition to physical absorption described above, chemical absorption can be employed. Gas containing large volumes of hydrogen sulfide can be freed from said hydrogen sulfide by first conducting the gas stream into aqueous solutions containing copper ions in water for absorbing the hydrogen sulfide and then oxidizing the copper sulfide thus formed with air or oxygen gas to produce elemental sulfur. DE 2304497 discloses an aqueous absorption medium which can contain high concentrations of copper ions (for example, 28.9 g Cu in 1400 ml water), and absorption of the hydrogen sulfide carried out by bubbling the gas into the aqueous medium.
EP 0 986 432 B1 discloses a method for selective hydrogen sulfide removal from gases comprising both H2S and CO2. When these components were present in the gas in CO2 to H2S ratio of 2:1, the method removed 99% of the H2S selectively. However, when said ratio was 200:1, the H2S removal was 95%.
U.S. Pat. No. 2,889,197 discloses a two-step gas washing method including two alkaline washes. Both washes are performed with alkali, ammonium is mentioned, and an inorganic salt. Disclosed is the recovery of the sulfur component from the wash solution as a sulfate suitable for use as a fertilizer. Column 3, lines 30-31, indicates that after these two alkaline washes, an acidic washing is employed. The document discloses no experimental proof on the effectiveness or results obtainable by said method.
It can be desirable to provide a method for removal of sulfur components and carbon dioxide from syngas obtainable by gasification of carbonaceous material, for example, when gasifying biomass. It can be desirable to remove sulfur components and carbon dioxide from the syngas in an energy efficient way. It can be desirable to effectively combine sulfur component and carbon dioxide removal. It can be desirable to provide simplification, increase of the effectiveness and identification of possibilities for synergism of the overall BTL process.